1. Field of the Invention
The present invention relates to a power supply unit for a pulse laser using a magnetic switch (saturable reactor) used for pulse discharging a laser, and more particularly to an improvement for lowering the peaking capacity of the pulse laser without lowering the energy per pulse of the laser pulse.
2. Description of the Related Art
As a pulse power supply unit for a high-power pulse laser and an accelerator, those using a magnetic pulse compression circuit are often used for the improvement of the durability of a main switch such as a thyratron and GTO these years.
FIG. 20 shows an equivalent circuit of a general capacity shift type of magnetic pulse compression device used for the pulse power supply of a pulse laser, and FIG. 21 shows an example of waveforms of the voltage and the current at respective points of the circuit shown in FIG. 20.
The discharge circuit of FIG. 20 is a two-step magnetic pulse compression circuit utilizing a saturation phenomenon of three magnetic switches AL0 to AL2 which consist of saturable reactors.
In FIG. 20, the electric charge is charged from high-voltage power supply HV to capacitor C0 through magnetic switch AL0 and coil L1.
Then, when pulse oscillation synchronizing signal (trigger signal) TR which turns on in synchronization with the repetition frequency of the pulse laser oscillation is input, main switch SW is turned on at this point (time t0 in FIG. 10). When the main switch SW is turned on, potential VSW of the main switch SW drops suddenly to zero, and then when time product (integration value of time of voltage VC0) S0 of voltage difference VC0-VSW between the capacitor C0 and the main switch SW which are both end voltages of the magnetic switch AL0 reaches a limit value decided depending on a set characteristic of the magnetic switch AL0, the magnetic switch AL0 is saturated at this time t1, and current pulse i0 flows through a loop of the capacitor C0, the magnetic switch AL0, the main switch SW and capacitor C1.
Time .delta.0 elapsed between the start of flowing the current pulse i0 and its termination to become zero (time t2), namely, electric charge transfer time .delta.0 elapsed to complete the transfer of the electric charge from the capacitor C0 to the capacitor C1, is determined by respective capacities of a post-inductance of the magnetic switch AL0, the capacitor C0 and the capacitor C1, provided that a loss by the main switch SW and the like is disregarded.
When time product S1 of voltage VC1 of the capacitor C1 reaches a limit value which is determined by a determined characteristic of the magnetic switch AL1, the magnetic switch AL1 saturates at this time t3 to have a low inductance. Thus, current pulse i1 flows through a loop of the capacitor C1, capacitor C2 and the magnetic switch AL1. This current pulse i1 becomes zero at time t4 after a lapse of predetermined transfer time .delta.1 which is determined by the capacities of the capacitors C1 and C2 and the post-saturation inductance of the magnetic switch AL1.
And, when time product S2 of voltage VC2 of the capacitor C2 reaches a limit value which is determined by a determined characteristic of the magnetic switch AL2, the magnetic switch AL2 is saturated at this time t5, and current pulse i2 flows through a loop of the capacitor C2, peaking capacitor CP and the magnetic switch AL2.
Then, voltage VCp of the peaking capacitor Cp rises with the progress of charging, and when this voltage VCp reaches a predetermined main discharge initiation voltage, a dielectric breakdown of the laser gas is caused between main electrodes 10 at this time t6 to start the main discharge. The laser medium is excited by this main discharge, and the laser light is generated in several nsec.
Then, the voltage of the peaking capacitor Cp drops rapidly by the main discharge and returns to the state before starting to charge after a lapse of predetermined time.
This electric discharge operation is repeated by the switching operation of the main switch SW which is synchronized with the trigger signal TR, and the pulse laser oscillation is performed at a fixed repetition frequency (pulse oscillation frequency).
In this case, since it is determined that the inductances of the charge transfer circuits in respective stages comprising the magnetic switch and the capacitor become small toward the final stage, the pulse compression operation is performed so that the peak value of current pulses i0 to i2 rises sequentially and the current-carrying width becomes small sequentially. As a result, an intense electric discharge is obtained between the main electrodes 6 in a short time.
When the pulse compression rate is excessively raised by the aforesaid magnetic pulse compression, there is obtained a pulse laser light having a short pulse width and high peak output. But, this pulse laser light having a short pulse width and high peak output causes various problems such as follows:
durability of optical parts installed in the laser resonator is degraded; PA1 a round trip frequency (shuttling frequency of the laser light in the resonator) is decreased; and PA1 an incidence frequency to a narrow-banding optical element decreases as the round trip frequency is decreased, and a narrow-banding efficiency drops. PA1 the saturable reactors are divided into parallel circuits of a plurality of saturable reactors, the plurality of saturable reactors are magnetically coupled, and post-saturation inductances of the plurality of saturable reactors are made different. PA1 the series circuit of the saturable reactors and the transferring capacitors is connected in a plurality of numbers in parallel to the peaking capacitor, and PA1 the plurality of saturable reactors are magnetically coupled, and saturable reactors for finely adjusting a transfer initiation time are connected in series to the plurality of saturable reactors.
Accordingly, the pulse laser light that its pulse width is not too short and its peak output is not too large is often demanded recently. Because the pulse width becomes long even if the peaking capacity of the pulse laser light is lowered, the energy per pulse of the laser pulse does not become small as compared with the pulse laser light having a short pulse width and a high peak output.
However, the aforesaid conventional technology is designed to transfer all the charges as current pulse i2 from the capacitor C2 to the peaking capacitor Cp at a time. Therefore, the luminescence intensity and the luminescence time of the pulse laser light are determined univocally depending on the electric charge transfer time and the peak value of the current pulse i2 only, and their fine adjustment is hardly made. The aforesaid conventional technology had a limitation in its circuit in increasing the pulse width of the laser pulse because the electric charge transferred from the capacitor C2 to the peaking capacitor Cp is mostly consumed by the electric discharge, and the electric charge is not transferred to the peaking capacitor Cp after the laser luminescence is started.
The present invention was completed in view of the aforesaid circumstances, and it is an object of the invention to provide a power supply unit for a pulse laser, which has a simple structure but can provide a pulse laser light that its pulse width is not too short and its peak output is not too large.